In the manufacture of electronic microcircuits the need arises in some integrated circuit fabrication processes to form a patterned resist layer on top of a previously formed patterned material layer where the patterned resist layer to be fabricated either has the same pattern as the underlying previously formed material pattern or has the complementary pattern thereof. A patterned resist layer of this kind may be used to either selectively cover or to selectively uncover an underlying patterned material layer for a subsequent selective treatment step. The treatment may involve material deposition, material removal, or material modification. For example, material may be selectively coated onto the patterned layer to build a desired laminate pattern using a lift-off technique. This might be used, for example where the thickness of a metal pattern needs to be increased or a surface characteristic changed for some reason. This need arises because thin metal patterns are most easily defined lithographically while thick metal patterns are more desired because they are electrically more conductive and form better masking patterns. The ability to easily and conveniently convert a thin metal pattern to a thick metal pattern is thus very useful.
One prior art method for accomplishing this step is to coat the previously formed pattern with a photoresist layer, align a suitably patterned photomask to the underlying pattern, expose the photoresist layer through the aligned mask, and then develop the exposed photoresist layer. If the photomask was identical with or the exact complement of the underlying material pattern, then the developed photoresist layer will either exactly cover or not cover the underlying pattern. One drawback of this prior art method is that a suitably patterned photomask is required. Another disadvantage is that precise alignment is required. If the underlying pattern has experienced a heat cycle after formation, distortion usually occurs, which makes exact overlay and accurate alignment impossible.
Where the underlying pattern is a metal and the treatment step consists of the addition of more metal, electroplating and electroless plating have also been used in the prior art. A disadvantage of electroplating is that all of the metal pattern parts must be electrically interconnected, a requirement which is not often met in practice. Electroless plating has other disadvantages, such as a tendency to have non-uniform plating thickness. In both cases, the parts of the structure not covered by the metal pattern are also exposed to the plating solution and thus become contaminated.